Hakone volcano is situated at the northern tip of the Izu-Mariana volcanic arc in eastern Japan, and area that is both tectonically and volcanically active. Fumarolic activity is observed a.t post-caldera cone volcanoes within the caldera, and the northern extension of the Kita-Izu fault, the source of a M7.3 earthquake in 1930 (Kita-Izu earthquake), traverses the southern part of the caldera. Although there is no historical record of eruptive activity, many intense earthquake swarms have been reported since 1786 within the caldera. In this study, literature on earthquake swarms in 1917, 1920, 1933-35, 1943, 1944, 1953, 1959-60 are re-examined to reveal detailed development of the activity, seismic intensity and the epicentral region of these events. Two epicentral regions are recognized; the central cones region (1), and the southern part of the caldera (2). Earthquake swarms in (1) are often accompanied by rumblings and the main shock is not distinct; successive earthquakes are felt almost continuously during the peak of activity. On the other hand, earthquake swarms in (2) are rarely accompanied by rumblings and have obvious sequence of foreshocks, a mainshock and aftershocks. The largest earthquakes in the swarms in (2) are larger than those in (1). The two epicentral regions are both on the northern extension of the Kita-Izu fault system. Differences in the style of earthquake swarm activity in regions (1) and (2) may be due to differences of the geology and the source depth. Correspondence between fumarolic activity in the solfataras at central cone volcanoes and seismic activity was not observed except for the 1933-35 swarm. Most of the earthquake swarms at Hakone volcano are therefore probably tectonic earthquakes on the Kita-Izu fault system rather than being related to hydrothermal or magmatic activity within the caldera. Earthquake swarms at Hakone appear to have been rare before 1917, and except for 1786, no historical records exist even though one of the most important highways in Japanese history passed across the volcano. An interpretation that attributes the earthquake swarms since 1917 to foreshocks and aftershocks of the 1930 Kita-Izu earthquake would broadly explain the frequency of earthquake swarms at Hakone volcano since the early twentieth century.

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Using commercial Windows PC and digital cameras, we have developed an automatic image recording system, which is inexpensive, small and lightweight. This system automatically records camera images in a digital form and transfers them to a remote server through telephone lines. This system can be flexibly applied to the observation and monitoring of eruption columns even under severe conditions. With this system we have receded images of the eruption columns of Miyakejima Volcano, Japan, every 10 seconds from 12 August 2000 to 13 May 2002. These images revealed how the vents of volcanic gas ejection developed during August 2000. The images after September 2000 showed that the column was relatively high in summer, probably reflecting the amount of vapor in the ambient atmosphere.

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Miocene igneous complexes in the Outer Zone of Southwest Japan intruded into the Shimanto accretionary prism of Cretaceous to Tertiary age, in association with tectonic events such as the opening of the Japan Sea and beginning of the subduction of the Shikoku Basin beneath the southwest Japan arc. A wide-band magnetotelluric survey was carried out in order to image the deep structure of the Kumano Acidic Rocks and the Omine Granitic Rocks which are large igneous complexes of Middle Miocene age in the Kii peninsula. The electrical resistivity structure of the area down to 30 km was estimated from the two dimensional inversion for the TM mode data. The electrical resistivity structure comprises a resistive layer of about 5 km thickness in the uppermost part of crust, a conductive layer at depths of 8-15 km beneath the Cretaceous sedimentary terrane, and a highly resistive body extended to the depth of about 20 km beneath the Miocene igneous complexes. Location and geometry of the highly resistive body are likely to indicate that the Kumano Acidic Rocks and the Omine Granitic Rocks are connected together in deeper subsurface. K-Ar and FT ages, geochemical compositions and other geophysical data support that the resistive body is composed of several fossil magma reservoirs in relation to the Kumano Acidic Rocks and the Omine Granitic Rocks.

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From May 8 to 13 in 2002, a small-scale earthquake swarm took place off Ito City, eastern Izu Peninsula, central Japan. Associated with the swarm, changes in the ground tilt as much as 2.8 μrad were detected by several tiltmeters near the swarm area. The change can be approximately interpreted by a tensile fault in the vicinity of the swarm area. We estimated the fault parameters from the tilt and GPS data, assuming that the fault length is 2 km. The depth of the upper bound of the tensile fault and the width are 10 km and 2 km, respectively. The estimated volumetric increase is 7 × 10^-3 km³. This tensile fault is confirmed to be a dike intrusion from a deep seated magma reservoir, that is, the same process which had been observed in 1980’s to 1990’s. The tilt vectors of stations ITO and YOS are quite similar in shape in the period from May 8 to 11, suggesting thickening of the dike without upward elongation during the period.

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We observed volcanic tremor associated with the Asama volcano eruption on February 6, 2003. The seismic activity of the Asama volcano began to increase at about 3 : 00, and the eruption occurred at 12 : 00. The volcanic tremor was preceded by one earthquake having large amplitude, and was followed by another. The source of the tremor, which is estimated by using amplitude decay with distance, is below the vent. Moreover, the tremor contained harmonic oscillation having a dominant frequency of 1.4 Hz. Amplitude of the harmonic oscillation increased just after the first large quake, and decreased just before the second one. Particle motions of the harmonic tremor point to the vent. We interpret the source mechanism of the harmonic tremor as a resonance of gas in the conduit.

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In order to find mechanical relations between caldera form and regional stress field, we approximated collapse of a magma chamber by contraction of a small sphere in the elastic medium and estimated distribution of the plastic and/or rupturing area due to the contraction of the small sphere by the Coulomb yield criterion under an assumption of the elastic-perfectly-plastic material. As a result, we found that the plastic area (caldera) showed a circle distribution on the surface when the regional stress field was not given or the isotropic regional stress field was given. On the other hand, when the anisotropic regional stress field was given, we found that the caldera enlarged toward the direction of the maximum compression axis (or the minimum extension axis), so that it showed an elliptic outline on the surface.

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The drilling core (501.9 m in depth) has been obtained from Okogashima Island located in southern margin of the Aira caldera at northern end of Kagoshima Bay in southern Kyushu. The core reveals detailed eruption history prior to the Aira pyroclastic eruptions about 25,000 years ago by descriptions of lithofacies and fissiontrack dating. The three types of lithofacies are identified in the drilling core. They are (1) 2-8 m in depth: pyroxene andesite; (2) 8-253.2 m in depth: the Okogashima rhyolite lava flow; (3) 253.2-501.9m in depth: pyroclastic materials. This study focuses mainly on pyroclastic materials, which have not been described yet. The pyroclastic materials in lower levels are composed mainly of tuffaceous sand with cross lamination, containing shell fossils. They also contain pumice-concentration zone and welded tuff, indicating explosive eruption. The pumice-concentration zone occurs in 326-340 m (unit A), 354-360 m (unit B), 425-427 m (unit D), 449-450 m (unit E), and 478-497 m (unit F), while the welded tuff occurs in 360-380 m (unit C). The fission-track ages of around 0.4 Ma from the units B and C indicate that the deposits were erupted prior to the Aira pyroclastic eruptions. Therefore, this drilling core shows that at least sixth pyroclastic eruptions are identified in southern margin of the caldera prior to the Aira pyroclastic eruptions.

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Kozushima is located on the Zenisu Ridge which is one of across-arc NE-SW oriented echelon ridges in Northern Izu Arc. The island consists of rhyolitic lava domes, thick lava flows and pyroclastic materials. The eruptive history of the island is not well known. Takodoyama Volcano is mainly composed of lava dome and is arranged on NW-SE line with Osawa Volcano and Matsuyamahana Volcano. Previous studies showed that these three volcanoes have been formed by the same eruption of 22,000-19,000 yrs BP (about 26-22 cal ka BP) and have deposited Chichibuyama pyroclastic surge deposit A (Cb-A) on Membo Volcano in southern Kozushima. However, this latter clarifies that the pyroclastic flow and surge deposits which appear to form the pyroclastic cone of the Takodoyama Volcano are overlain by AT ash (25,000 yrs BP; about 29 cal ka BP) and are correlated with Chichibuyama pyroclastic surge deposit B (Cb-B) on Membo Volcano. Chemical compositions of the pumice lapilli and essential block in the pyroclastic deposits are the same as those of rhyolitic lavas from Takodoyama Volcano. These data show the pyroclastic deposits and Cb-B on Membo Volcano are derived from Takodoyama Volcano, and Takodoyama Volcano was formed before the deposition of AT ash and is older than Osawa Volcano and Matsuyamahana Volcano.

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JERS-1 SAR interferometry is applied to Kirishima volcanic complex, and we detected the ground deformation associated with the subsidence at the summit of Iwo-yama. The mean deformation rate for the analyzed three pairs of data was apparently constant for about 3.4 years. By inverting the obtained line-of-sight displacement field for each pair of data, we estimate the parameters of pressure source by using the Mogi model. Estimated depth of the source is about 270m, which is consistent with the depth of aquifer suggested from MT survey. Hence, this subsidence could be caused by the volume change of the aquifer system beneath the Iwo-yama. The rate of volume change was about - 1.3× 10⁴ m³/yr.

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During the Taisho Eruption (1914-1915) of Sakurajima Volcano, the Secondary Lava Flows drained out from the front of the primary lava flows after a certain pause from their settlement. We investigate distribution, timing and conditions of the effusion of the Secondary Lava Flows which have not been clarified yet. We define the features of the Secondary Lava Flows by means of geomorphological interpretation using aerial photographs and field survey. More than 200 landscape photographs, which were taken at the time of the Taisho Eruption, indicate that the earliest Secondary Lava Flows effused on 14 February 1914; this date was several months earlier than those previously considered. The effusing of the Secondary Lava Flows did not occur as a single event but as several events in different areas. Based on volumetric estimation of the individual Secondary Lava Flows, the volume of the Secondary Lava Flows tend to have increased as time passed after the eruption. The Secondary Lava Flows effused from the underlying tip of river valleys.

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